This invention relates to a method for controlling incineration in a combustor for low-level radio active wastes, and in more detail relates to a method for economizing the consumption of supplemental fuel while maintaining the incineration state in the combustor stable, by controlling the amount of said supplemental fuel and the amount of said radioactive wastes to be fed to the combustor.
Combustible low-level radioactive wastes exhausted from nuclear energy plants and atomic energy laboratories include polythylene sheets, paper, cloths, timber and the like. It has been usually known that on disposal of these wastes, it is most effective to incinerate the wastes through the aid of a liquid fuel such as kerosene, light oil, LNG, LPG or waste oil, and reduce their volume. On incineration of the radioactive wastes, there is generally known a combustor that is maintained at a negative static pressure and normally comprises three combustion chambers maintained in an excess air environment. And, the wastes are burned as they pass through the combustion chambers in a cyclonic flow pattern. FIG. 1 is a cross-sectional view illustrating one example of the combustor for radioactive wastes. In FIG. 1, a first combustion chamber 2 of a combustor 1 is fed with primary air, supplemental fuel and radioactive wastes respectively from three inlets 3, 4, 6, and further with secondary air from an inlet 5, thereby forming a cyclonic flow. At this stage, the wastes are ignited and move to a second combustion chamber 7. In said second combustion chamber 7, most combustible wastes are completely incinerated, and their ashes fall to the bottom of the combustor. Some wastes, which are more difficult to burn, are incinerated at the bottom of the combustor. A third combustion chamber 8 partitioned from the second combustion chamber 7 by a wall is a zone designed to complete the removal of the heavier particles from the flue gas and provide sufficient residence time for complete combustion.
In the case of incinerating radioactive wastes in the combustor, as a matter of couse, it is preferable to incinerate the wastes completely by consuming a small amount of supplemental fuel and maintaining the incineration state in the combustor stable during the period of incineration. Accordingly, the prior art has employed the method for maintaining the outlet gas temperature of the combustor at a certain target value as shown in FIG. 1, which comprise the steps of detecting an outlet gas temperature of the combustor by means of a thermometer 9, sending its signal to a temperature-indicator controller (TIC) 10, driving for instance a diaphragm valve 11 by the outlet of said controller and decreasing or increasing the amount of supplemental fuel fed to the combustor, wherein 815.degree..+-.15.degree. C. is generally selected as said target value. This method for controlling incineration is designed to maintain the amount of wastes fed to the combustor per unit time uniform, and increase or reduce the supply amount of supplemental fuel in correspondence with rise and fall of the outlet gas temperature of the combustor. This method is effective for the purpose of maintaining the incineration state stable, so far as the heating value of wastes fed to the combustor is unchanged.
However, the radioactive wasts to be incinerated are of various kinds, and are also different in the points of heating value and bulk density depending upon their kinds. In this connection, it is to be noted that when comparing paper with polyethylene, the former is inferior in the heating value and bulk density to the latter. Accordingly, the usual incineration control method is disadvantageous in that when wastes are fed to the combustor in a uniform amount per unit time, for instance said wastes being consisted mainly of paper, complete incineration of wastes can be achieved even when the supply amount of supplemental fuel is small, but as the heating value of wastes is small, needless supplemental fuel is fed to the combustor and consumed. In case the wastes to be incinerated are consisted mainly of polyethylene, the usual incineration control method encounters different problems such as fusion of ashes and the like due to the fact that since its heating value and bulk density are conspicuously large as compared with paper, the heating value generated in the combustor also becomes extremely large, thereby making it difficult to maintain a normal incineration state. In addition, this incineration control method is disadvantageous in that since every time when the construction of wastes fed to the combustor is changed, the heating value thereof is also changed, hunting takes place every time when the construction of wastes fed to the combustor is changed, and so it becomes extremely difficult to maintain the complete incineration state stable.
On the other hand, there has been used the incineration control method for maintaining the outlet gas temperature of the combustor uniform which comprises setting the amount of supplemental fuel fed to the combustor, and controlling the amount of wastes fed to the combustor alone, wherein, however, since the burning rate of the wastes is slower than that of supplemental fuel, a long time delay will occur until a change in the supply amount of wastes will cause a change in the outlet gas temperature of the combustor. Therefore, this incineration control method is liable to cause hunting, and so can never incinerate the wastes stably and completely.